docs: def corrected_key(self, key):
          """return the matching valid key, if ``key.lower()`` is a unique
          starting sequence to identify the valid key, ``else None``
  
          """
          matching_keys = []
          for allowed_key in CMAOptions.defaults():
              if allowed_key.lower() == key.lower():
                  return allowed_key
              if allowed_key.lower().startswith(key.lower()):
                  matching_keys.append(allowed_key)
          return matching_keys[0] if len(matching_keys) == 1 else None